When the driver of the vehicle pushes the clutch pedal down (in a vehicle having a clutch of the type defined above) the pedal actuates a clutch fork or declutching fork which applies the said actuating member, usually a clutch release bearing, on the free ends of the diaphragm fingers. This causes the diaphragm to tilt, so that it no longer exerts any force on the pressure plate. In consequence, the pressure plate, which is usually biased by means of return tongues, releases the clutch disc so that it is no longer coupled in rotation to the engine flywheel and the gearbox is therefore uncoupled from the engine.
In one known design, the outer peripheral portion of the diaphragm is frustoconical in its free state and is formed from sheet metal having a substantially constant thickness. It then cooperates firstly with an annular land formed on the face of the pressure plate in facing relationship with it, and secondly with another annular land which is formed in facing relationship with it on the internal face of the cover plate.
Using a conventional elastic diaphragm according to that design, the resilient force which is exerted axially by the diaphragm on the pressure plate depends essentially on the thickness of the metal from which the diaphragm is formed, and on the dimensions of the outer peripheral portion of the latter, which are determined by the angle of the frustum of a cone defined by the diaphragm.
As the travel of the clutch release bearing (or course of deformation of the diaphragm fingers) progresses, the resilient force passes through a maximum which can be quite pronounced, before again decreasing. The ratio of the depth (measured axially) of the peripheral portion of the diaphragm to the thickness of the diaphragm determines the shape of this characteristic curve of the diaphragm.
It is possible to reduce the height of the summit or peak of the curve giving the value of the elastic force as a function of the degree of deformation of the diaphragm, in particular by increasing the thickness of the metal in the diaphragm. However, the reliability and fatigue resistance of such a type of diaphragm, when made of thick metal, are then both reduced. In addition, a very thick diaphragm is difficult to manufacture by press forming, and this also increases the total weight of the clutch.
Another disadvantage of clutches of conventional design lies in the fact that the pressure plate and cover plate have to be made with axially projecting annular lands, which increases the complexity of their design and also their manufacturing costs.
It has previously been proposed, in United Kingdom patents specification GB 2158183A and the corresponding French published patent specification FR 2563877A, to reduce the maximum height of the characteristic curve of the diaphragm by making the latter in the form of an annular resilient diaphragm having a body in the form of a disc. In its unstressed state, this disc has a substantially frustoconical shape, and comprises a body of revolution defining at least one annular profiled portion which is centred on the axis of the cone and which has a concavity directed towards the exterior of the frustum of a cone. However, this is not an entirely satisfactory solution, firstly because any interference between the profiled portion of the pressure plate must be avoided, and secondly because it is still necessary to provide an annular land projecting axially on the internal face of the cover plate, and another land on the pressure plate.